1. Field of the Invention
The present invention relates to an FM modulation circuit for use in a video modulator or the like, and more particularly to an improved FM modulation circuit of high modulation degree and reduced modulation distortion.
2. Description of the Prior Art
A known form of FM modulation circuit is shown in FIG. 1 and comprises an oscillation circuit and a variable capacitance circuit. In the oscillation circuit, a resonance circuit consisting of a capacitor 11 and a coil 13 is connected to the collector of a transistor 12 for oscillation, a feedback capacitor 14 is connected between the collector and emitter of the transistor 12, and bias resistors 8, 9 and 17 are disposed as shown. Shown at numerals 10 and 15 are capacitors. The variable capacitance circuit is connected to the resonance circuit of the oscillation circuit, and a voice signal applied to the variable capacitance circuit varies the capacitance thereof, whereby the oscillation frequency of the oscillation circuit is varied in correspondence with the voice signal.
The variable capacitance circuit enclosed with broken lines in FIG. 1 has a varactor diode 6 which is given an appropriate bias by means of resistors 4 and 5. The voice signal is applied from an input terminal 1 to the varactor diode 6 through a capacitor 2 and a resistor 3 so as to vary the capacitance of the varactor diode 6 in correspondence with the voice signal. The variable capacitance circuit and the oscillation circuit are coupled through a capacitor 7 having a capacitance on the order of 130 pF. The FM output is delivered from the oscillation circuit to a terminal 18 via a capacitor 16.
The FM modulation circuit as stated above employs the varactor diode in the variable capacitance circuit, and therefore has the following disadvantages. When the bias voltage is made small in order to raise the modulation degree, the variation in the capacitance is not linear and the modulation distortion increases. On the other hand, when the bias voltage is made great in order to reduce the modulation distortion, the capacitance variation is small and the modulation degree lowers.